In U.S. Pat. Nos. 4,184,014, 4,093,784 and 4,042,756 and in German Pat. No. 2,262,256 there are described electrochemical cells having an alkali metal or alkaline earth metal anode, an oxyhalide as cathode and as solvent for an electrolyte salt, an electrolyte salt comprising a lithium cation and a current collector.
The lithium-thionyl chloride cell has the highest energy density of commercially available primary cells. It has a very good low- and high-temperature performance, excellent shelf life and a very high power density in its high-rate configuration. However, the major obstacle to the commercialization of a high-rate type cell, and in particular, a high-rate multicell battery is its well known explosion hazard during reversal or charging. This hazard is common to all high-rate nonaqueous lithium batteries. Reversal of one or more cells or the charging of one row of cells by another row connected in parallel is likely to happen during deep discharge. The reason for this is that lithium is deposited at low over-potentials, on itself, or on a passivated cathode. The metallic lithium which deposits under these conditions is likely to have a large active surface area. At a high deposition rate, a powder like lithium deposit can be formed. The presence of lithium powder in a thionyl-chloride cell can create a very hazardous condition. On prolonged reversals or charging of a lithium cell there is the danger of an internal short circuit due to lithium dendrite growth. This may lead to internal spark which can rapidly melt, or even evaporate, the lithium dendrite, thereby initiating a cell explosion. This explosion is more likely to happen in a partially discharged cell where less electrolyte is available to cool the spark area.
Calcium has a much higher melting point (838.degree. C.) than lithium (180.5.degree. C.). Therefore, the probability that an internal spark will lead to evaporation or melting of calcium is much smaller.
The activation time for a reserve cell of this type ought to be as short as possible. An activation time of 1 second for small batteries and cells, and of about 5 seconds for large batteries is acceptable. U.S. Pat. No. 4,150,198 discloses a reserve cell comprising an inorganic oxyhalide solvent and a Lewis acid solute dissolved therein as the sole salt with an oxidizable active anode, which is preferably lithium. The present invention overcomes the safety problem of cells of the prior art and it has a shorter activation time.